1. Field of the invention
The invention generally relates to a method and device for introducing (installing) cables, such as e.g. optical glass fibre cables, into a channelization system of guide tubes, in general designated as "ducts", which are disposed, as a rule in the ground, according to a predetermined arrangement.
2. State of the art
According to a usual technique cables are installed by utilizing tensile force. The course of the required tensile force, which has to remain smaller than a permissible value in connection with the mechanical properties of the cable, depends on the geometry of the channelization system (in which as a rule curves and/or windings occur), on the friction forces between the cable and the duct, and on the properties of the cable. Without taking into account the stiffness of the cable the following items are of importance for the required tensile force: the friction forces consequent on the mass of the cable, and the friction forces caused by the tensile stress occurring in the cable (in connection with the curves and/or windings of a relevant duct portion). The former friction forces give rise to a tensile force which increases linearly with the length of cable installed. The latter friction forces give rise to a tensile force which increases exponentially with the number of curves or windings. This imposes an important restriction on the maximum cable length which can be installed in one go. Because of the exponential increase of the required tensile force it can be tried to see to it that the cable tension will be as low as possible. This involves that the friction forces caused by the mass of the cable have to be compensated locally. This means that the installation force has to be exerted divided over the whole length of a cable section to be installed. In a method serving for that purpose a flow of compressed gas (compressed air) is effected from the inlet end of a relevant duct and directed to the outlet end of the same, a relevant cable being introduced into the inlet end. A similar method is known from the European patent No. 0108590. This patent also discloses a device for employing such a method, which device is provided with a cable injection unit with in it a hollow, substantially rectilinear cable lead-through channel with an entrance end and an exit end for leading in and leading out a cable which has to be introduced into the relevant duct, which cable injection unit is further provided with a gas channel, which debouches into the cable lead-through channel and via which compressed gas can be supplied to the cable lead-through channel, as well as with a set of wheels mounted opposite to each other and partly reaching into the lead-through channel, and serving to move on a cable, disposed between these wheels and touching them, in the direction of the exit end. This known technique is meant for installing lightweight, flexible optical fibre cables. The flow of compressed air led through a duct exerts a drag force working on a cable introduced into the duct, due to which such a cable will be dragged through the duct to its outlet end. As appears from this known technique the velocity of the flow of compressed air is practically linearly dependent (increasing) on the difference in pressure there is between the inlet end and the outlet end of a relevant duct portion. The length of the duct portion over which a cable section can be installed at one processing stroke is in this case limited to abt. 200 m, even though it is expected in the above-mentioned patent specification that said length can be extended to abt. 300 m for a cable with a weight of 3 gms/m and when the difference in pressure is abt. 55 psi. The two wheels forming part of a cable injection unit according to the aforesaid patent specification exclusively serve to compensate the opposing forces exerted on the cable and caused by the difference in pressure there is between the interior and the outer environments of the injection unit.